An internal gear is the one with the teeth formed on the inner surface of a cylinder or cone, with teeth that are parallel to the axis of rotation. It provides a compact parallel shaft transmission drive with large speed reduction, used with a stock spur pinion the ratio is the same as the two external gears. Internal gear can be cut as either spur or helical, it can provide high ratios and smooth running. Internal gears are designed to transmit motion and power between parallel shafts rotates in the same direction.

The advantages of the internal gears are reduced sliding action, which leads to less friction, less tooth wear and the slippage of the teeth is less.

Internal gear are used in some of the industries like automobiles, sugar mills, textile plants, material handling equipments, paper mills, thermal power stations. Some of the materials used are alloy steel, carbon steel, bronze and plastic.